This invention relates to the decoding of binary-coded transmissions in which the digits are coded by variations in effective pulse length. An example of such a code is the biphase-mark code.
In a biphase-mark code the digits 0 and 1 are transmitted by pulses of different lengths, the pulse length for a one being half the duration of the pulse length for a zero. The digits are however transmitted at a steady rate, so two short pulses are actually transmitted to indicate a one and one long pulse is transmitted to indicate a zero. A pulse in this context can be positive-going or negative going, and the beginning of a pulse is defined by the existence of a change in polarity. A biphase-mark code is illustrated in FIG. 1.
It can be seen from FIG. 1 that each bit to be transmitted can be said to be defined by two consecutive binary states. The first state of a symbol is always different from the second state of the previous symbol. To transmit a zero, the second state of the symbol is the same as the first state of the same symbol. To transmit a one, the second state of the symbol is different from the first state of the same symbol. The code has the advantage of having no (or constant) mean d.c. value.
At a receiver such a code may be decoded by measurement of the length of the pulses. A one will thus be decoded as two short-length pulses, and a zero will be decoded as one long pulse. The critical discriminating factor between zeros and ones is the length of the pulses.
The biphase-mark code has been specified for the transmission of two-channel, serial digital audio data within broadcasting installations, in AES 3.1985, now incorporated in IEC 958. To support most common sampling rates, plus a margin for varispeed, it may be desired to decode signals received with a wide range of sample rates, for example from 28 KHz to 54 kHz. The corresponding bit rate is 64 times the sample rate. The code is also used in consumer digital audio applications. The signal is not accompanied by any separate synchronising or timing signal, and thus the receiver/decoder has to determine the sample rate itself, over a range which is virtually 2:1, using only the pulses themselves. The signal may have to travel over several hundred metres of cable which further distorts the timing of the transitions and hence the length of pulses received. This requirement presents substantial problems.